My thoughts on the matter:
1) Windows Defender (on Win7) is NOT an anti-VIRUS program, rather, it is an anti-MALWARE program. So that maxim that "running two REALTIME antivirus programs may lead to conflicts and/or a decrease in performance" does not apply here.
2) Some have contended that even running two [realtime] anti-MALWARE programs isn't in one's best interest, again citing conflicts and/or decrease in performance. If that's the case, then why do many avast users successfully run (and recommend running) MBAM (MalwareBytes Anti-Malware) --- in realtime--- alongside avast???
3) The argument that WD isn't a good choice... not the best quality... that there are better programs out there (such as MBAM???)... is just that: a USER CHOICE. And any user who wishes to do so... whether that's in his/her best interest or not... should have their choice respected [unless there's indisputable causal evidence of inflicting harm, which I don't believe there is].
4) Many Win7 users who keep WD around do so merely for a "second opinion", as an "on-demand" scanner. That removes the issue of a REAL-TIME conflict. If so, what is the remaining objection to an on-demand scanner? (That "there are better options" is again a matter of personal choice, not a matter of inflicting damage to one's system.)
5) If the contention is that WD contains a large segment Microsoft's anti-malware database, again raising the argument of conflict, then it would seem to follow that avast users will risk the same conflict when running Microsoft's Malicious Software Removal Tool each month as part of Windows Updates... The MSRT scan is VERY similar to an on-demand scan by WD. Is the next step for avast to block the MSRT as well? Or advise us that it's a pointless duplication of what avast is already protecting?
6) Finally, as others have noted, avast had allowed us to run WD, for years now, alongside avast... doing so was never an issue. What happened, NOW, to change things? If there's not an essential reason for this change, I too would like to see things changed back to the way they were. In particular, it's bad enough for avast to block WD once... but for it to repeatedly change back the directory entry after a user manually resets it is definitely taking things too far.